1. Field of the Invention
The invention relates to an anti-skid device for motor vehicles, particularly to an anti-skid device for motor vehicles which comprises a first valve disposed in a hydraulic circuit and selectively dividing the hydraulic circuit connecting a brake master cylinder with a wheelbrake cylinder into a master cylinder side hydraulic circuit and a wheelbrake side hydraulic circuit; a hydraulic pressure source generating working fluid pressure by a driving force; a pressure reducing piston for opening and closing the first valve by a forward movement from its inactive position caused by hydraulic pressure of the wheelbrake cylinder side hydraulic circuit and a backward movement to its inactive position caused by the working fluid pressure and thereby increasing or decreasing the capacity of the wheelbrake cylinder side hydraulic circuit; and a second valve for supplying the working fluid pressure to the pressure reducing piston when the wheel is rotating in a normal condition in order to make the piston to be in its inactive position against the hydraulic pressure of the wheelbrake cylinder and for releasing the working fluid pressure from the pressure reducing piston when the wheel is about to be locked in order to move the piston forward by means of the hydraulic pressure of the wheelbrake cylinder side hydraulic circuit, and said anti-skid device is so constructed that the driver of the vehicle can recognize the actuation of the anti-skid device through a vibration of brake-pedal which generates according to the change of hydraulic pressure degree at the wheelbrake cylinder side hydraulic circuit when the anti-skid device is actuated.
2. Prior Art
The conventional anti-skid device of the type which is constructed so that a brake-pedal vibrates when the anti-skid device is actuated and thereby the driver can recognize the actuation of the anti-skid device, is disclosed, for example, in Japanese Laid-open Patent Specification No. 79450/l984 (Jitsu-kai-sho 59-79450). In the device disclosed in the above specification, a brake master cylinder and a wheel brake cylinder are hydraulically connected by an interposed floating cylinder, and a working fluid chamber is formed by the floating piston. When the anti-skid device is actuated, the capacity of the wheel brake cylinder side hydraulic circuit is increased and decreased by supplying to or taking from the floating piston the working fluid pressure, thereby the hydraulic pressure in the wheel brake cylinder side hydraulic circuit being controlled so as to be increased and decreased. Consequently, at the time of actuation of the anti-skid device, the capacity of the brake master cylinder side hydraulic circuit varies by a sliding movement of the floating piston, which causes the brake-pedal to generate vibration.
In the above conventional device, however, the sliding degree of the floating piston has to be set so as to satisfy the control efficiency of the hydraulic pressure of wheel brake cylinder side. This might cause a problem that the vibration amplitude of brake-pedal becomes too large due to the excessive change of the capacity of the master cylinder side hydraulic circuit caused by the sliding movement of the floating piston.